Stroke related deaths constituted more than 150,000 deaths annually in the U.S. in 2004 and many more deaths on a global scale. Stroke is the third leading cause of death, ranking behind “diseases of the heart” and all forms of cancer. Stroke is also a leading cause of serious, long-term disability in the United States. A significant portion of deaths related to cardiovascular problems are attributed to carotid artery disease. In human anatomy, the common carotid artery is an artery that supplies the head and neck with oxygenated blood; it divides in the neck to form the external and internal carotid arteries.
Ischemic stroke occurs when an artery to the brain is blocked. The brain depends on its arteries to bring fresh blood from the heart and lungs. The blood carries oxygen and nutrients to the brain, and removes carbon dioxide and cellular waste from the blood. If an artery is blocked, the brain cells (neurons) cannot produce enough energy and will eventually cease function. If the artery remains blocked for more than a few minutes, the brain cells may die. Immediate medical treatment is absolutely critical after the onset of ischemic stroke.
Ischemic stroke can be caused by several different kinds of diseases. The most common cause is narrowing of the arteries in the neck or head. Narrowing of the arteries in the neck or head is most often caused atherosclerosis, or gradual cholesterol deposition. If the arteries become too narrow, blood cells may collect and form blood clots. These blood clots can block the artery where the blood blots are formed (thrombosis), or the blood clots can dislodge and become trapped in arteries closer to the brain (embolism). Another cause of stroke is blood clots located in the heart, which can occur as a result of irregular heartbeat (for example, atrial fibrillation), heart attack, or abnormalities of the heart valves. While these are the most common causes of ischemic stroke, there are many other possible causes. Other examples include use of street drugs, traumatic injury to the blood vessels of the neck, or disorders of blood clotting.
Carotid artery disease occurs when one or both arteries become narrowed or blocked by a buildup of plaque. Plaque is made up of scar tissue, blood cells in the artery wall, cholesterol, and other fatty substances. This plaque buildup results in hardening of the arteries, otherwise known as atherosclerosis. Atherosclerosis can slow or stop blood flow to the brain. As a result, the brain may not receive enough oxygen, sugar, and other nutrients carried by the blood and needed for brain-cell activities.
In carotid artery disease, the chief culprit is the build up of plaque, specifically soft-plaque, in the carotid arteries. Accordingly, there is high interest in the medical community in detecting carotid artery disease.
Typically, soft-plaque is not easily detectable in X-ray images or non-contrasted computed tomography (CT) images. In comparison, calcified plaque is much more readily detectable than soft plaque and thus the presence of hard plaque has been used as a surrogate for the presence of soft plaque, with the reasoning being that calcified plaque is the by product of ruptured soft plaque.
Carotid plaque can be classified into six stages according to the Stary scale. According to general medical consensus, detecting the presence of plaque in stage 4 and stage 5 is very important because stages 4 and 5 constitute critical vulnerable plaque which could lead to rupture or dislodging of the plaque causing blockages, which in turn could lead to myocardial infarction (MCI), MCI being commonly known as “heart attack.”
A conventional medical imaging technique for determining plaque and constituency of the plaque is intravascular ultrasound (IVUS). However IVUS is only performed on symptomatic patients due to the invasive nature of IVUS. Symptomatic patients are already at an advanced stage and past non-invasive therapy options.
With the advent of cardiac volume computed tomography (VCT) and the ever increasing spatial and temporal resolution of IVUS and with the impending advent of high definition (HD) VCT, imaging a contrasted study of heat that is gated to mitigate heart motion is within reasonable reach. Using IVUS images and HD VCT images, plaque can be distinguished from lumen and plaque can be distinguished from calcification. However distinguishing plaque from lumen and calcification is still very difficult in an automated manner.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for non-invasive detection of soft carotid plaque at stages earlier than stage 4 and stage 5. There is also a need for improved method of distinguishing carotoid artery plaque from lumen and distinguishing carotoid artery plaque from calcification in an automated manner.